In recent years, many biopharmaceuticals have been released into the market. Most of such pharmaceuticals are products of recombinant proteins obtained by introducing genes into animal cells. Technologies for efficient production of such recombinant proteins in animal cells enable biopharmaceutical cost reduction and promise a stable supply to patients.
Conventionally, elongation factor 1 (EF1) α promoter, SRα promoter, and the like were used in such expression vectors (Patent Document 1; Non-Patent Documents 1 and 2). Recently, as a result of further improvements for efficient expression, strong promoters such as the human CMV enhancer combined with chicken β-actin promoter (CAG promoter) and human CMV enhancer combined with human EF1α promoter (CEF promoter) have been constructed and used for the purpose described above (Non-Patent Documents 3 to 5).
However, the need to develop vectors having stronger promoters is still remained. Such vectors are expected to contribute greatly to a further reduction of costs and a more stable supply of biopharmaceuticals.    Patent Document 1: WO92/19759    Non-Patent Document 1: Mol. Cell. Biol., Vol. 8 (1), p. 466-472, 1988    Non-Patent Document 2: Gene, Vol. 87(2), p. 291-294, 1990    Non-Patent Document 3: Gene, Vol. 272, p. 149-156, 2001    Non-Patent Document 4: Analytical Biochemistry, Vol. 247, p. 179-181, 1997    Non-Patent Document 5: Gene, Vol. 108, p. 193-200, 1991